1. Field of the Invention:
The present invention relates to apparatus for the noncontact measurement of the surface profile of a steeply spherical surface at low magnification ( 10X). More particularly, the invention relates to optical apparatus which is useful for the high accuracy measurement of surface roughness of a steep spherical surface or of the deviation from sphericity of a steeply spherical surface. 2. The Prior Art
Prior art techniques available for measuring, with high precision, the surface of a steeply spherical surface include mechanical and optical profilers. A commonly used contacting apparatus used to measure surface profiles is a stylus instrument, e.g., the commercially available Talysurf or the Talystep. However, in the case of a soft or delicate surface, the stylus digs into the surface and measurement results do not truly represent the surface. Other limitations of the stylus technique include its high sensitivity to microphonics and vibrations, the delicate nature of the stylus and the mechanism, and the need for a highly skilled operator to align and use it.
Prior art optical profilers have been based on a variety of techniques, e.g., scanning fringes of equal chromatic order (FECO) interferometry, see for example, J.M. Bennett, "Measurement of the RMS Roughness, Autocovariance Function and Other Statistical Properties of Optical Surfaces using a FECO Scanning Interferometer," Applied Optics, Vol. 15, pp. 2705-2721 (1976); scanning Fizeau interferometry, see for example J.M. Eastman and P.W. Baumeister, "Measurement of the Microtopography of Optical Surfaces using a Scanning Fizeau Interferometer," J. Opt. Soc. Am., Vol. 64, p. l369(A) (1974); optical heterodyne interferometry, see for example, G.E. Sommargren, "Optical Heterodyne Profilometry," Applied Optics, Vol. 20, pp 610-618 (1981); a Mirau interferometer, see for example, B. Bhushan, J.C. Wyant, and C.L. Koliopoulis, "Measurement of Surface Topography of Magnetic Tapes by Mirau Interferometry," Applied Optics, Vol. 24, pp. 1489-1497 (1985) and J.C. Wyant and K.N. Prettyjohns, U.S. Pat. No. 4,639,139, issued Jan. 27, 1987; a Nomarski-based instrument, see for example S.N. Jabr, "Surface-roughness Measurement by Digital Processing of Nomarski phase contrast Images," Optics Letters, Vol. 10, pp. 526-528 (1985); a birefringent microscope, see for example, M.J. Downs, U.S. Pat. No. 4,534,649, issued Aug. 13, 1985; and shearing interference microscopy, see for example, M. Adachi and K. Yasaka, "Roughness measurement using a shearing interference microscope," Applied Optics, Vol. 25, pp. 764-768 (1986).
FECO interferometry requires that the test surface be brought very close to the reference surface, e.g., typically within about several micrometers, thereby frequently causing the test surface to be damaged by residual dust particles. Also, FECO interferometry does not lend itself easily to the measurement of spherical surfaces.
The optical heterodyne interferometer, which is both common path and does not require a reference surface, produces very accurate and precise measurements. While this technique provides state-of-the-art optical measurements, it suffers from a number of limitations. In particular, the apparatus is complex and expensive. In addition, since the technique only scans in a circle of fixed radius, it does not profile an area of the test surface. Finally, the out of plane measurement capability is severely restricted to a quarter of the illumination wavelength so that only planar or near planar surfaces can be measured.
The conventional white light or filtered white light Mirau type two-beam interferometer microscope suffers from several serious limitations. The conventional Mirau obscuring reference surface precludes using the interferometer technique with low power objective magnifications whether the reference surface is spherical or planar. Because of the obscuring reference surface an incoherent illumination source must be used which severly limits the usable depth of focus of the objective to the coherence length of the illumination source, see J.F. Biegen, U.S. Pat. No. 4,732,483, issued Mar. 22, 1987. Also, the vertical alignment tolerance needed for fringe acquisition is on the order of a few mirometers.
Other two-beam, equal path interferometer microscopes such as the Michelson and Linnik are adaptable to utilizing spherical reference surfaces for low magnification objectives. The disadvantage of the Michelson is the serious reduction of the objective working distance which results from the necessary positioning of a beamsplitter cube to the front of the objective. The Linnik interferometer technique of using two objectives is both expensive and difficult to align. The Michelson and Linnik, when used with the extended incoherent illumination source, suffer the same tight vertical alignment tolerance as does the Mirau interferometer microscope.
The birefringent microscope technique is both common path and does not require a reference surface. However, it does have some severe limitations. First, it only scans a line so that it does not profile an area of the test surface. Second, it is limited in its ability to use a sufficiently large diameter for the reference beam on the test surface, thereby limiting the extent to which lower spatial frequencies, such as spherical test surfaces, can be measured.
In the present invention, high precision profile measurements can be made of spherical test surfaces using low power objectives ( 10X) with a Mirau type two-beam microscope interferometer. The interference of the two-beam microscope interferometer is localized within the full depth of focus of every objective magnification permitting a large vertical alignment tolerance. A large range of test surface curvatures from plano to steeply spherical can be measured relative to the prior art. The improvements of the present invention, thusly, overcome the disadvantages of the prior art and allow the high accuracy, fine lateral resolution measurement of spherical surface microroughness profiles and deviations from sphericity.